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Home Brew Forums > Home Brewing Beer > Brew Science > Does RO sparge water need to be acidified?
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Old 02-06-2013, 01:03 AM   #11
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I think a lot of people may be doing calculations based on the assumption that if you have alkalinity of 50 you need to neutralize all of it to get to mash pH.
I think that might be done for the simplicity of it. While i think the exact approach is more correct and should be implemented in water calculators it is easy to argue that the difference of 12% is in the noise of all the other factors that determine pH in mash and sparge.

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Old 02-06-2013, 01:27 AM   #12
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Yes but it changes with pH. My well water runs pH 6.4 usually and has alkalinity of 80. If I want to bring that to pH 5.5, to use the same example, I need 1.24 mEq/L of acid which, compared to the 1.6 mEq/L alkalinity is 77.5% for 22.5% error. I think that amount is OK for ROM mental calculations but 22.5 out of 100 is only 13 dB signal to noise ratio. Not so impressive.

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Old 02-06-2013, 02:00 AM   #13
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Here is a question while the topic is open. I just installed an RO system in my house the other day and my pH readings on the water are 5.4... I was expecting something closer to 7. Does 5.4 sound right or is there something wrong with the installation?
Its OK. Gases will permeate the membrane very easily. The primary gas in water is CO2. So you end up with a lot of CO2 in the product water of RO systems. We often send the product water through air stripping towers to help get that excess CO2 out of the water so that the pH isn't crazy as you've observed.
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Old 02-06-2013, 02:13 AM   #14
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Its OK. Gases will permeate the membrane very easily. The primary gas in water is CO2. So you end up with a lot of CO2 in the product water of RO systems. We often send the product water through air stripping towers to help get that excess CO2 out of the water so that the pH isn't crazy as you've observed.
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Everything is cool. RO water has little buffering capacity (alkalinity). It is like a small capacitor. If you put any charge at all in it its pH will change quite a bit. There is a bit of CO2 in the air. It is quite soluble in water and when it dissolves it forms carbonic acid which, as the name suggests is a proton donor. The tiny amount in the air and the tiny amount of that which dissolves and the tiny fraction of that which actually emits protons is nevertheless sufficient to lower the pH of the water into the 5's or 6's.
That is interesting stuff. Thanks guys.

So when you say "put charge in it" do you mean the action of pumping the water from the holding tank? So the water in the tank would likely be a higher pH before it comes out the spigot? Or is this occurring within the membrane itself? I'm just asking out of curiosity. I can brew with pH 5.4 water just fine.
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Old 02-06-2013, 05:34 AM   #15
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No. That phrase was drawn from the analogy with the capacitor. In chemistry terms I am saying that it takes very few protons, i.e. very little dissolved CO2 to shift the pH of the water to these lower pH's.

Once the water is exposed to air it will immediately pick up CO2 and become acidified. Or, if the water comes from a well in a region with nominal rainfall that water will contain dissolved carbonic at a higher concentration that water that is in equilibrium with the air. I have no idea what the rejection for carbonic acid molecules might be but I suppose it is less that 100% so some of the CO2 in the water could be from the source. In any event it will eventually equilibrate with the air especially if it is accumulated in an atmospheric tank. I also vaguely recall reading that hydrogen ions are rejected less than hydroxyl ions so that there is some decrease in pH from that effect (if I'm remembering that right).

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